This invention arose from an effort to develop a fast response humidity sensor for data required to calculate the rate of evaporation of water from a crop or soil surface. It followed initial development of a fast-response, ultrasonic anemometer which accurately measures the vertical component of the wind. When this anemometer was used with a fine-wire thermometer, one could determine the departures of temperature and vertical wind from their mean values. By accumulating the product of these departures over a time period one could effectively measure the flux of heat above crop and soil surfaces.
A logical extension of this instrumentation was the development of a fast-response humidity sensor. Fluctuations in atmospheric moisture, multiplied by fluctuations in vertical wind, provide a calculation of the rate of evaporation of water from a crop or soil surface. However, the humidity sensor used in such measurements must have a fast-response, be very sensitive to changes in atmospheric moisture, operate reliably from battery power so as to be portable, be sufficiently durable for field operations, and have a stable calibration. Several humidity sensors had been designed to measure water vapor fluctuations, but none of them met all of these requirements.
It was concluded that an ultraviolet hygrometer, using a hydrogen source tube and a nitric oxide detector, would come closest to meeting the requirements listed above. However, no commercially available instrument could be operated by use of battery power.
These project needs led to the design of a low power Lyman-alpha hygrometer. The background of such instruments is generally described in the following three articles, which are hereby incorporated within this disclosure by reference:
"The Variable-Path Lyman-alpha Hygrometer and Its Operating Characteristics", by Arden L. Buck. Bulletin of the American Meteorological Society, Volume 57, No. 9, September, 1976, at pages 1113-1118.
"Water Vapor Measurements Utilizing the Absorption of Vacuum Ultraviolet and Infrared Radiation", by James E. Tillman; published in Principles and Methods of Measuring Humidity in Gases, pages 428-443, R. E. Ruskin, Editor.
"Humidity Fluctuations Over a Vegetated Surface Measured with a Lyman-alpha Hygrometer and a Fine-Wire Thermocouple Psychrometer" by T. Grayson Redford, Jr. et al, Journal of Applied Meteorology, Volume 19, July, 1980, pages 860-867.
As a result of this development, a low power Lyman-alpha hygrometer was designed, using commercial source and detector tubes and a conventional, open path arrangement so that air could move freely between the source and detector. The open path was between 0.5 and 1.0 cm in length, depending upon ambient moisture conditions. It is commercially available from Campbell Scientific, Inc. of Logan, Utah and is identified as Model 220. Its operation is discussed in the "Lyman-alpha Hygrometer Operator's Manual" published by Campbell Scientific, Inc., which is also hereby incorporated within this disclosure by reference.
The Model 220 Lyman-alpha Hygrometer has been sufficiently sensitive for accurate measurement purposes, has adequately fast-response, and operates satisfactorily from a battery power source. While this hygrometer has been in successful operation since its design in 1978, inherent shortcomings in its components have prompted continual further research and development. Most serious was the fact that calibration of the instrument was not stable over time. In addition, the source and detector tubes originally used in the instrument were not adequately reliable or rugged. They often required replacement during the course of experiments, at considerable expense in both time and money.
There were two reasons for these problems. One was that the seals between the magnesium fluoride windows and the glass tubes used as the source and detector were sometimes defective. While this is a particularly difficult seal to construct, the problem was cured by locating a source of tubes incorporating a special glass frit seal. This cured the problems of tube field reliability and durability. However, the calibration difficulties and degradation of source tube strength were inherent in the use of hydrogen as the filling gas in the source tube.
Hydrogen, in the excited state, is very reactive. During use of the hygrometer, the hydrogen emits ultraviolet radiation at the Lyman-alpha wavelength and also reacts chemically with the electrodes. It is therefore used up over a period of time. In order to extend tube life, they often are filled with a greater supply of hydrogen than is necessary for optimum performance. This higher gas pressure inside the tube contaminates the resulting spectrum with radiation at many undesired wavelengths in addition to the desired Lyman-alpha line. Such contamination produces results that do not follow Beer's simple law, which makes the calibration of the instrument then more complex.
We were able to deal with these complexities, and to obtain usable calibration data. However, the most serious difficulty encountered remained the fact that as the hydrogen was used, the radiation spectrum changed, resulting in changes in instrument calibration. The hygrometer was usable, but not ideal.
Two approaches have been tried by others to correct this operating deficiency inherent in all Lyman-alpha hygrometers. In the article by Buck, cited above, there is a description of the use of uranium hydride in the hydrogen source tube. When heated to the appropriate operating temperature, the presence of uranium hydride controls the hydrogen pressure inside the tube to maintain it at the correct value so that a very pure Lyman-alpha line is emitted. The hydride replaces hydrogen as it is used up at the electrodes. However, this approach presented two problems in the design of our portable hygrometer. First, the tubes were very expensive. Second, the heater needed to control the temperature of the hydride used too much power for battery operation.
The second known approach is to excite the hydrogen by using microwaves from outside the tube, thereby eliminating the need for electrodes within the tube. While such tubes are commercially available and do emit very pure Lyman-alpha radiation, they are very expensive and again consume too much power for a portable unit.
The published article by Tillman, cited above, discusses experimental use of a buffered glow discharge consisting of hydrogen and an inert buffer gas, such as argon. This was proposed as a method of reducing undesirable emissions in the hydrogen spectrum. The article states that the buffered discharge is more difficult to regulate and that it probably would operate at a lower hydrogen pressure and consequently have a shorter life due to the "cleanup" of the hydrogen.
To our knowledge, all ultraviolet hygrometers have used hydrogen as the radiation source. More specifically, they use the Lyman-alpha line, which is an emission line of atomic hydrogen found in the far ultraviolet region at 121.56 nm. This line is attractive for hygrometry because of the very strong absorption by water vapor at that wavelength. The absorption of the radiation by water at the Lyman-alpha emission wavelength is several hundred times greater than the corresponding absorption of the radiation by the oxygen in the air. The large absorption coefficient for water vapor results in a significant fraction of the radiation being absorbed within a few millimeters in a hygrometer light path, and a very fast response which can be accurately measured.
All of the literature references relating to spectroscopic hygrometry emphasize the ideal match between the discrimination ratios for absorption of water and oxygen at the Lyman-alpha emission line wavelength and infer that no other emission line is of similar practical value. The present invention arose from experimental substitution of a krypton-filled source tube for the hydrogen tube in order to capitalize on the purity of the krypton emission spectrum and long tube life. Krypton tubes have been available as calibration sources. Because krypton is inert, the gas within the tube does not deteriorate substantially over the tube life.
The substitution of krypton for hydrogen in the hygrometer resulted in new and unexpected results, namely accurate humidity measurements achievable through use of a low power source tube. The tube has long life and can be more accurately calibrated because of its inherent linear operation.